The present invention relates to a coextruded, biaxially stretch-oriented, opaque polyolefinic multilayered film comprising an opaque core layer and two transparent top layers.
Within the scope of this disclosure, the term "opaque" is to be understood as defining the property of the respective layer to scatter visible light.
From the state of the art, quite a number of opaque polyolefinic films are known, which either are single-layered films and are commonly employed as synthetic paper or comprise an opaque carrier film and one or two transparent top layers applied thereto. As an example of the numerous publications concerning opaque films, reference is made to DE-B-28 14 311.
It is one common feature of the known opaque films that their opaque appearance results from the fact that due to the inert fillers contained in the opaque layer, in combination with the biaxial stretching under defined process conditions, a great number of micro-cavities (vacuoles) are produced within the opaque layer, where the visible light penetrating through the layer is refracted. During the stretching of the film, the polymer matrix is torn at the peripheries of the grains of inorganic and/or organic fillers which are incompatible with the polyolefin, which leads to the formation of the mentioned vacuoles, i.e., of free (empty, unfilled) spaces. Of course, these vacuoles or micro-cavities are distributed over the whole volume of the filler-containing layer.
Due to their structure, all known opaque films possess a more or less pronounced tendency towards delamination, i.e., they show little strength in the level of the opaque layer. For a few special applications, this readiness to delamination of the opaque layer is quite advantageous (see EP-A-0,093,370), but in many other fields of application the delamination tendency of opaque films is highly undesirable and should be avoided.